U.S. Provisional Application 62/183,771 filed on Jun. 24, 2015 is hereby incorporated by reference in its entirety.
In U.S. Pat. No. 7,335,921 filed May 8, 2003, U.S. patent application Ser. No. 10/434,941 filed May 8, 2003, and U.S. Pat. No. 9,129,552 filed May 8, 2003 light emitting diode devices (LEDs) and particularly organic light emitting diode devices (OLEDs) are described in which one or more photonic crystal structures are integrated with the light emitting diode structures so as to provide enhanced levels of light emission and energy efficiency through the phenomenon of stimulated emission. The devices described include both laser and non-laser devices. The principle of operation of these devices, that have been collectively termed feedback enhanced organic light emitting diodes (FE-OLEDs), is that light is fed back into the light emitting layers of the devices from the photonic crystals, or if only one photonic crystal from other reflective structures so as to yield stimulated emission of light from the light emitting layers. Since the direction of propagation of the feedback light is vertical to the plane of these devices, and since light produced by stimulated emission propagates in the same direction as the light stimulating the emission, nearly all light emitted from FE-OLEDs is emitted in the direction perpendicular to the plane of the device. That is to say, the devices are vertically emitting.
The nearly complete vertical emission of light in the FE-OLEDs results in greatly increased efficiency of light generation versus power input because light is not emitted with a substantially in-plane direction of propagation. Light that is emitted with a substantially in-plane direction of propagation in OLEDs and LEDs is trapped within the devices by reflection from layer to layer interfaces within the devices and eventually absorbed in the devices creating heat. Thus the vertical emission of light in FE-OLEDs results in improved energy efficiency.
FE-OLEDs are generally described as either Type 1 or Type 2. The first (Type 1 device) can be characterized as operating by means of light generation within a defect in a one-dimensional photonic crystal. Photonic crystals are dielectric media that have a periodic variation of refractive index of light through their extent. A result of the periodic variation of refractive index is that the wave equation for light propagation has no solutions over a range of wavelengths (the stop band) for light propagating in the direction(s) of periodic index variation. A result is that a light emitting material molecule embedded in the photonic crystal medium cannot emit light having wavelengths within the stop band in the direction(s) of periodic index variation. A second result is that light propagating external to the photonic crystal medium will be completely reflected from the medium's surface if it has a wavelength within the stop band and if it is traveling in the direction or in one of the directions of refractive index variation. The photonic crystal structures utilized in FE-OLEDs are usually one-dimensional in nature with the axis of refractive index variation perpendicular to the planes of the devices, in other words it is parallel to the transmission axis of the device, but higher dimensional photonic crystals may also be used.
In a Type 1 device there is a planar region (a defect) constituting a cavity within the photonic crystal medium and parallel to the plane of the device in which the regular cyclic variation of refractive index does not occur. Another way of describing this structure is that there is a phase-slip in the photonic crystal medium that produces a defect. If a light emitting molecule is embedded within the defect, the light that it emits within the wavelength stop band will be trapped within the defect by the reflective properties of the photonic crystal medium. The thickness of the defect can be made quite small with the result that the photon density of light trapped in the defect can become quite high. This results in very efficient stimulation of light emission from excited state molecules embedded in the material in the defect.
Viewed classically the defect in a Type 1 device is a micro-cavity between two photonic crystal reflectors. Light produced by emitter molecules in the micro-cavity is reflected back and forth between the two reflectors stimulating more light emission.
While the Type 1 devices take advantage of the light reflection properties of photonic crystal structures, Type 2 devices depend on the properties of light emitted inside photonic crystals. As described above, a stop band is created inside a photonic crystal structure. This is a range of wavelengths over which there are no propagation of light allowed through the cyclically varying refractive index structure. It would be tempting to think that the light propagation modes or states that exist in the stop band wavelengths in free space are destroyed when the photonic crystal structure is introduced around the light emitter. However, this is not the case. What actually happens is that the light propagation states are expelled from the stop band and are “stacked up” at its edges in the wavelength spectrum. This sort of behavior is generally quantified in terms of the density of states, that is to say the number of allowable wave propagation states or modes per interval of energy in the electromagnetic spectrum. Plots of the density of states versus light frequency for free space (dashed line) and for a photonic crystal medium (solid line) are shown in FIG. 1. In the stop band wavelength region between frequencies B and C, the density of states is zero. However, at wavelengths A and D that are slightly red and blue shifted respectively from the boundaries of the stop band, this “stacking up” phenomenon means that the densities of states in the photonic crystal medium are considerably higher than for light propagating in free space. Since the amount of light that an emitting molecule will emit into its surrounding depends on the density of states available to propagate the light, a light emitter in the photonic crystal of FIG. 1 will emit considerably more light at frequencies A and D than it would in free space.
A further property of photonic crystals is that at wavelengths (or frequencies) close to the stop band, the medium is partially transmissive and partially reflective. The result is that light emitted at these wavelengths builds up within the medium as it is localized about the point of emission. The combination of the higher than normal level of photon emission at wavelengths adjacent to the stop band combined with buildup of these photons within the medium due to internal reflections yields very high photon densities throughout the bulk of the photonic crystal medium. These high photon densities in turn ensure stimulated emission from essentially all the excited state emitter molecules embedded in the photonic crystal. Because of the one-dimensional nature of the photonic crystal structure used, the propagation direction of the light produced by stimulated emission is all in the vertical direction, just as it was in the Type 1 devices, the energy efficiency of the Type 2 devices is similarly enhanced.
Laser devices have been demonstrated in the form of chiral lasers utilizing a chiral liquid crystal doped with a fluorescent dye. The structure of an aligned chiral liquid crystal is shown in FIG. 2. Rod shaped liquid crystal molecules 210 within layers 220 tend to align with their long axes pointing in single direction parallel to an axis called the director 230. In aligned nematic liquid crystals the director always points in the same direction. However, because of the asymmetric shape of some of the molecular constituents, in chiral liquid crystals as one passes down through the material along axis 240 as shown in FIG. 2, the direction of the director 230 rotates so as to sweep out a helix. Thus the individual liquid crystal molecules combine into a helical structure. Because of the anisotropy of electronic polarization within the individual molecules, light of the proper circular polarization passing through the liquid crystalline material encounters a much higher refractive index when its associated electric vector is parallel to the molecules' long axes than when its electric vector is perpendicular to the molecules long axes. Given that the light has the proper wavelength, light of the opposite circular polarization (for instance, right-handed versus left-handed) has its associated electric vector rotated so as to track the helical structure of the liquid crystal and thus sees no change in refractive index as it passes up through the chiral liquid crystalline structure. Thus circularly polarized light of the correct handedness propagating parallel to axis 240 sees a periodically oscillating refractive index as it passes through molecules with their long axes oscillating between directions parallel and anti-parallel to the light's electric vector. In this way the assembly of chiral liquid crystalline molecules act as a one-dimensional photonic crystal material for one circular polarization of light along axis 240. The medium will have a stop band for light emission similar to that shown in FIG. 1 and there will be enhancement of the density of states near the stop band edges in the electromagnetic spectrum. If the fluorescent dye doped into this aligned chiral liquid crystal is excited to emit light (for instance, by optical pumping with light from a YAG laser) and the light produced has in its spectral band a wavelength in the chiral medium the same as one of the wavelengths at the stop band edges (analogous to A and D in FIG. 1) a buildup of photon density will occur at that band edge wavelength. These photons will stimulate further emission. In the chiral laser there is more than sufficient stimulated emission and little enough absorptive losses to enable lasing. These optically pumped dye doped liquid crystal lasers operate on a similar principle as a Type 2 solid state device except that light emission is excited by optical excitation rather than electrical excitation as in a light emitting diode. Another important aspect of the chiral liquid crystal laser devices is that the fluorescent emitter material was embedded throughout the chiral liquid crystal medium. This yielded a wide enough distribution of fluorescent material to produce sufficient laser gain to initiate lasing.
Producing a solid state Type 2 device encounters immediate issues stemming from the structures of conventional LEDs and organic light emitting diodes (OLEDs) an example 300 of which is portrayed in FIG. 3. The OLED 300 consists of a substrate 310, a transparent anode 320, a hole injection layer 330, a hole transporting layer 340, an emitter layer 350, an electron transporting layer 360, and a metal cathode 370. The device 300 functions as follows: when an electrical potential difference is applied between the anode and the cathode, positively charged holes are injected from the anode 320 into the hole injection layer 330. Under the influence of the imposed electric field the holes flow from the hole injection layer, through the hole transporting layer 340 and into the emitter layer 350. At the same time electrons are injected from the cathode 370 into the electron transporting layer 360. Under the influence of the imposed electric field the electrons flow from the electron transporting layer into the emitter layer 350. In the emitter layer the electrons and holes pair together on single organic molecules promoting the molecules into electronically excited states. These excited states (excitons) then collapse to produce light.
In OLEDs the hole injection layers 330, the hole transporting layers 340, the emitter layers 350, and the electron transporting layers 360 are all composed of organic materials, which have quite low charge carrier (electron or hole) mobilities as compared to typical inorganic semiconductor materials. In current OLEDs the total thickness of all the organic layers is less than 200 nanometers and the emitter layers have thicknesses in the range of 25 to 50 nanometers.
Inorganic Type 1 devices in which the emitter layer is located in a defect or micro-cavity between two photonic crystal structures are described in U.S. Pat. No. 4,949,350 which teaches Type 1 devices in which the light emitting material is an inorganic semiconductor such as In0.2Ga0.8As. An example of the structure of these devices is portrayed in FIG. 4 and FIG. 5. The photonic crystal structures above and below the cavity containing an emitter consist of epitaxially grown alternating layers of AlAs (lower refractive index) and GaAs (higher refractive index) with layer thicknesses corresponding to one-quarter wave thickness for light with the wavelength of 900+ nm. wavelength desired from the device.
FIG. 4 presents a top level view of the device 400. The device consists of an transparent electron conducting substrate 410, an electron conducting photonic crystal mirror 420, a spacer layer 430 that performs much the same function as an electron transporting layer in OLEDs, an emitter layer 440, a second spacer layer 450 that performs much the same function as a hole transporting layer, a hole conducting photonic crystal layer 460, a gold anode, and electrical contacts 480 and 490.
FIG. 5 presents a more detailed view of layers 420 and 430. The electron conducting photonic crystal mirror 420 consists of twenty pairs 510 of an aluminum arsenide 520 and a gallium arsenide 540 layer. Between each layer pair is a “superlattice” 530 consisting of four extremely thin (1.7 nanometers) alternating GaAS and AlAs layers. These superlattices are included to improve electrical conductivity and have no optical function. Also shown in FIG. 5 are the sub-layers that compose the first spacer layer 430. These are an ALAs layer 550, another superlattice 560, and a GaAS layer 570. Also shown is a graded index layer 580 that is intended to further confine light into the emitter layer. Photonic crystal layer 460 is also built up from alternating AlAS and GaAs layers just as photonic crystal layer 420 was. In this example twelve layer pairs of these materials were used.
Referring back to FIG. 4, when the device is energized, electrons from substrate 410 pass through the intervening layers into emitter layer 440. At the same time holes from the anode 470 pass through the intervening layers into emitter layer 440. The electrons and holes recombine in the emitter layer to produce infrared light. This light is reflected off the two photonic crystal stacks and the photon density builds up in the cavity consisting of layers 430, 440 and 450. This high photon density produces stimulated emission at a sufficient level that lasing is induced. Devices like that shown in FIG. 4 have become known as VCSELs (vertical cavity, surface emitting lasers) and are now commonly available.
U.S. Pat. No. 6,160,828 describes an organic Type 1 device known as a VCSEL an example of which, 600, is portrayed in FIG. 6. The device consists of a transparent substrate 610, a first mirror layer 620, a first electrode 630, a hole transporting layer 640, an emitter layer 650, an electron transporting layer 660, a second electrode 670, and a second mirror layer 680. Layers 640, 650, and 660 are composed of organic materials. The patent states that the first mirror layer and/or the second mirror layer are distributed Bragg reflectors. Distributed Bragg reflectors are one-dimensional photonic crystal structures built up by successive deposition of layers of alternating high and low refractive index. As was the case in the VCSELs described in U.S. Pat. No. 4,949,350, electrons and holes recombine in the emitter layer 650 of this device producing light that is then reflected by the mirror layers back through the emitter layers stimulating further light emission. A high enough photon density is built up in the cavity between the mirrors to initiate lasing
What both U.S. Pat. Nos. 4,949,350 and 6,160,828 have in common is that the photonic crystal structures disclosed as being used on either side of the cavity that confines light in the laser cavities are formed by successive deposition of many layers of light transmitting material with alternating high and low refractive indices. In the case of OLEDs the fabrication techniques used to fabricate these devices are vacuum evaporation or sputtering. In order to produce efficiently working devices, it was necessary to deposit these very thin layers and the very thin layers within the laser micro-cavities with a high degree of accuracy in terms of thickness.
FE-OLEDs in which the photonic crystal structures are built up by successive deposition of layers of transparent material have photonic crystal structures in which the refractive index profile is discontinuous. That is to say the refractive index changes abruptly and discontinuously at layer boundaries in the photonic crystal structure. This layered structure requires accurate vacuum deposition of many layers, which becomes impractical if low manufacturing costs are required or as devices become larger in size. Therefore, a similar device having a simpler method of fabrication is desired.